1. Field of the Invention
The present invention relates to optical disk apparatus, and more particularly to optical disk apparatus which employ a semiconductor laser source to produce a plurality of light spots on an optical disk, thereby enabling the performing of overwriting and verification immediately after recording almost at the same time.
Further, the present invention relates to optical disk apparatus, and more particularly to optical recording and/or reproducing apparatus which employ a semiconductor laser source to produce a plurality of light spots on an optical disk, thereby enabling the performing of recording and reproduction in parallel.
2. Related Background Art
Recently, research and development has been active in improving the transfer rate of a magneto-optical disk apparatus. The magneto-optical disk apparatus presently commercially available require three rotations of the disk for erasing, recording, and reproducing (verifying) in writing data, thus having a drawback that the data transfer rate, especially in recording, is lower than hard disks and the like. Then there are proposed magneto-optical disk apparatus and magneto-optical disks which are overwritable and which can perform erasing and recording during a rotation of a disk. Also proposed are magneto-optical disk apparatus which can perform verification immediately after recording with a plurality of light spots, or apparatus which can perform recording and reproducing in parallel with a plurality of light spots.
As an example of the overwritable magneto-optical disk apparatus, as disclosed in Japanese Laid-open Patent Application No. 51-107121, there is a method for modulating a magnetic field applied to a magneto-optical recording medium in accordance with recording information. In order to improve the data transfer rate of magneto-optical disk apparatus, another proposal was given, for example in Japanese Laid-Open Patent Application No. 64-82348, which discloses an optical disk apparatus in which, in addition to the above, a plurality of light spots are provided for recording and for verifying on a track to enable the performing of erasing, recording, and reproducing (verifying) during a rotation of a disk. Since these are arranged to modulate the magnetic field applied to the magneto-optical recording medium in accordance with recording information, they are called a magnetic field modulation overwriting method.
Also, there are magneto-optical recording media which are overwritable by modulating a light beam for writing data into a magneto-optical recording medium, for example, as suggested in Japanese Laid-open Patent Applications No. 62-75948, No. 63-268103, etc. These recording media are overwritable because of their structure in which multilayer magnetic films having different characteristics of their Curie temperature or coercive force are exchange-coupled. These are called a light intensity modulation overwriting method.
Further, there are apparatus which employ a plurality of light sources to provide a plurality of light spots on adjacent tracks on a recording medium, thereby performing recording and reproducing in parallel, for example, as suggested in Japanese Laid-open Patent Applications No. 54-146613, No. 64-19535, etc. Using a semiconductor laser array as a light source, these achieve parallel recording and reproducing even using substantially the same optical system as in the conventional magneto-optical disk apparatus employing a single light source.
Since magneto-optical disk apparatus utilize the property that the optical system can propagate a plurality of light beams in a multiplexed manner as described, it is to be expected that the data transfer rate can be improved to be comparable with that of a hard disk apparatus or greater than that.
Incidentally, where the data transfer rate of magneto-optical disk apparatus is improved by such an arrangement that a plurality of laser light sources are used to perform overwriting and verification immediately after recording with a rotation of a disk or by such an arrangement that recording and reproduction are processed in parallel over a plurality of tracks, it is necessary that the plurality of light spots be arranged to accurately track a predetermined track or predetermined tracks thereon.
There are optical disk apparatus utilizing an operation of an image-rotating prism in order to keep a plurality of light spots accurately tracking on a track, as suggested in Japanese Laid-open Patent Application No. 1-177510, etc.
A conventional example is described by referring to FIG. 1. Reference numeral 1 designates a magneto-optical recording medium exhibiting a magneto-optical effect, provided on a disk. Also provided is a transparent substrate 3, and a protection film 2 thereof. A beam emitted from a semiconductor laser 4 is collimated by a collimator lens 5 and the collimated beam is separated by a diffraction grating 6 into a plurality of beams (three beams of the zeroth order and .+-.first order). The beams incident into a dove prism 7 are then reflected by an internal surface thereof and then enter a polarization beam splitter 9. Numeral 8 denotes an actuator for rotating the dove prism 7 about the optical axis. The beam reflected by a mirror 10 are focused by an objective lens 11 to form three light spots 14, 15, 16 on a predetermined track on the magneto-optical recording medium 1. Numeral 12 denotes an actuator for focusing and tracking, which drives the objective lens 11.
FIG. 2 shows the light spots on the magneto-optical recording medium 1 and the intensities of the respective light spots upon recording and upon reproduction. The central light spot 15 is zeroth order light while the peripheral light spots 14 and 16 are the .+-.first order defracted light, as aligned as shown on a track on the recording medium 1.
Taking the direction of rotation of the disk to be along the direction of the arrow, the light spot 15 is a recording/erasing light spot, and the light spot 16 is a light spot for verification immediately after recording. A ratio between the intensities of the respective light spots can be changed by the diffraction grating 6. For example, a ratio can be 1 mW for verification relative to 7 mw for recording and erasing.
FIG. 2 also includes emission power of the laser light source in respective processes of reproduction and recording. The laser is arranged to emit light at a low power Pr upon reproduction, so that the light spot 15 on this occasion is 1 mw to be used for reproduction of magneto-optical signals or for the detection of servo signals. The optical spot 16 is not used because of the low power thereof. Upon recording the laser is arranged to emit light at high power Pw. On this occasion the light spot 15 functions as a recording/erasing light spot. With irradiation of the laser light of high power, the temperature of recording medium 1 increases so as to lower the magnetization and the coercive force thereof. Thus, applying a magnetic field polarity-inverted in accordance with recording information by means of a magnetic head 13, magneto-optical pits are recorded. On the other hand, the light spot 16 has the reproduction power, and performs an error check immediately after recording while reproducing the magnetic-optical signals.
In FIG. 1, after being reflected by the magneto-optical recording medium 1 and again entering the objective lens 11, the beams travel to the mirror 10 and are then reflected by the polarization beam splitter 9 to be guided to a signal detection system.
FIG. 1 shows a differential detection system using a half wave plate 21 and a polarization beam splitter 22. Beams passing through the polarization beam splitter 22 are guided through a condenser lens 23 and a cylindrical lens 24 onto a photodetector 25. Beams reflected by the polarization beam splitter 22 are guided through a condenser lens 25 onto a photodetector 27. Reproduction of the magneto-optical signals is achieved using differential outputs (not shown) from the photodetectors 25 and 27. In FIG. 1, numeral 17 denotes a drive circuit for the semiconductor laser 4, 18 denotes a drive circuit for the actuator 8, 19 denotes a drive circuit for the actuator 12, 20 denotes a drive circuit for the magnetic head 13, 28 denotes a circuit for detecting magneto-optical signals and servo signals, and 29 denotes a controller.
Next described, referring to FIG. 3, is a detection system for detecting the servo signals, especially, tracing signals. FIG. 3 shows a state where the condenser lens 26 condenses the beams reflected by the polarization beam splitter 22 to form light spots 30-1, 30-2, and 30-3 on corresponding photodetectors 27-1, 27-2, and 27-3. The light spot 30-2 corresponds to the recording/erasing light spot 15 on the recording medium, and the light spot 30-3 corresponds to the verifying light spot 16.
Upon overwriting, two light spots need to be laid on a track on the recording medium. Then tracking is carried out in the ordinary push-pull method with the recording/erasing light spot 15. Outputs from the light spot 30-2 on the bisectional photodetector 27-2 are put through a differential amplifier 31 into the actuator drive circuit 19 to carry out tracking by the actuator 12 for the objective lens 11.
Next, in order to correct rotation in the plane of the recording medium, of the recording/erasing light spot 15 and the verifying light spot 16, differential outputs between the push-pull outputs of the light spots 30-1 and 30-3 are used. Since the differential outputs are low frequency outputs indicating an amount of rotation of the light spots 14 and 16 relative to a track, they are put into the actuator drive circuit 18 to perform rotation servo of light spots by the actuator 8 for rotating the dove prism. According to these procedures, the recording/erasing light spot 15 and verifying light spot 16 can be laid correctly on the same track within predetermined accuracy upon overwriting. For normal reproduction, the tracking of the push-pull method is simply carried out using only the light spot 15.
The above conventional example, however, requires the dove prism 7 and image-rotating acutator 8 in order to maintain the predetermined tracking accuracy for two light spots and, in addition, the three light spots on the photodetectors require precise positioning in the tracking direction, thus causing an increase in costs. Also, the structure of the optical head is complicated, thus making compact designing difficult.
In view of the problems in the above conventional example, an object of the present invention is to provide an optical head which needs neither a dove prism nor an image-rotating actuator, which is cheap and compact, and which achieves the predetermined tracking accuracy for a plurality of light spots, and also to provide an optical recording and/or reproducing apparatus using it.
Causes of track deviation of the verifying light spot 16 were investigated while effecting the tracking servo operation of the ordinary push-pull method only with the recording/erasing light spot 15. It was found by the investigation that a dominating factor was the positional deviation in the track direction between the center of rotation of the disk and the optical spot.
This will be described by using FIG. 4 and FIGS. 5A and 5B. In FIG. 4, reference numeral 34 designates a magneto-optical disk and 35 denotes a track on the disk. The magneto-optical disk 34 is loaded on a spindle motor 36 to rotate about the center O. Numeral 37 denotes a movable unit of an optical head, and numerals 11 and 15 denote an objective lens and a recording/erasing light spot, respectively. There are the objective lens 11, the actuator 12 for driving it, and the mirror 10 set in the optical head movable unit 37. In order to reduce the access time to a target track, the movable unit is composed of minimum components, and the heavy semiconductor laser source and signal detection system are set in a stationary unit 40 of the optical head.
Such a construction of the optical head is called a separate optical system, in which the optical head movable unit 37 moves in parallel with the radial direction O-O' of magneto-optical disk 34, for example on P-P', so as not to cause an optical axis deviation when it moves from the inner periphery (radius R1) to the outer periphery (radius R2) of disk 34. Numerals 11', 15', 37' represent the objective lens, the recording/erasing light spot, and the optical head movable unit, respectively, having moved to the outer peripheral portion of magneto-optical disk 34.
With a single light spot, the positional deviation in the track direction between the center of rotation of the disk and the light spot, i.e., the distance .DELTA. between O-O' and P-P', rarely matters. This distance .DELTA. does, however, matter for the optical head which is arranged to lay a plurality of light spots on the track for verification or the like immediately after recording.
This is next described referring to FIGS. 5A and 5B. They are enlarged drawings near the track 35 in FIG. 4 where the three light spots are focused. The zeroth order light and .+-.first order diffracted light separated by the diffraction grating 6 is focused as the light spots 15 and 14, 16, respectively, on a track of disk 34. The light spot 15 is used for recording or erasing and the light spot 16 for verification. The arrow 38 represents the direction of rotation of the disk.
For example, suppose the light spots 15 and 16 are set on a same track at the inner peripheral position of the disk (radius R1), as shown in FIG. 5A. A line connecting the two light spots is perpendicular to a line connecting the rotation center O of magneto-optical disk 34 with the light spot 15 in FIG. 4. If the optical head movable unit 37 is moved in this state to the outer peripheral position (radius R2), it is seen, as shown in FIG. 5B, that a difference in curvature between an inner peripheral track and an outer peripheral track causes the light spot 16 to have a detrack .delta. when the light spot 15 is set on the track. The detrack .delta. can be expressed by the formula below with a distance d between the two light spots. In the formula, R1 is an innermost radial position of movement of the plural light spots from the rotation center of concentric tracks and R2 an outermost radial position of movement of the plural light spots from the rotation center of concentric tracks. EQU .delta.=d.multidot..DELTA..multidot.(R2-R1)/R1.multidot.R2)(1)
Applying this to a 3.5-inch optical disk and letting R1=24 mm, R2=40 mm, for example, d=0.02 mm, and .DELTA.=1 mm, then .delta.=0.33 .mu.m, which is an unignorable amount as compared with the track pitch=1.6 .mu.m. The conventional example compensated for the detrack using a complicated mechanism including the dove prism and image-rotating actuator, which increased the costs because of the need for precise positioning of three light spots on the photodetectors in the tracking direction, as described previously. Also, the structure of the optical head was complicated, thus making compact designing difficult.
Therefore, the present invention has achieved the above object by the following arrangements of the optical recording/reproducing apparatus.